The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc., which performs a print outputting function for any purpose. Printers using intermediate transfer, transfix, or transfuse members are well known. In general, such printing systems typically include a printing or imaging member in combination with a printhead which is used to form an image on the imaging member. A final receiving surface or print medium is brought into contact with the imaging surface after the image has been placed thereon by the nozzles of the printhead. The image is then transferred and fixed to the print medium by the imaging member in combination with a transfix pressure member, or in other embodiments, by a separate fuser and pressure member.
Some printer systems which incorporate intermediate transfix members also incorporate a phase change ink. In one such printer system, the imaging process begins by applying a thin liquid, such as, for example, silicone oil, to an imaging member surface. The solid or hot melt ink is placed into a heated reservoir where it is melted into a liquid state. The highly engineered hot melt ink is formulated to meet a number of constraints, including low viscosity at jetting temperatures, specific visco-elastic properties at component-to-media transfer temperatures, and high durability at room temperatures.
The heated reservoir provides the liquefied ink to an associated printhead. Once within the printhead, the liquid ink flows through manifolds and is ejected from microscopic orifices through use of proprietary piezoelectric transducer (PZT) printhead technology. The duration and amplitude of the electrical pulse applied to the PZT is very accurately controlled so that a repeatable and precise pressure pulse can be applied to the ink resulting in the proper volume, velocity, and trajectory of the droplet. Several rows of jets, for example four rows, can be used, each one with a different color. The individual droplets of ink are jetted onto the liquid layer on the imaging member. The imaging member and liquid layer are held at a specific temperature at which the ink hardens to a ductile visco-elastic state.
In conjunction with forming the image on the imaging drum, a print medium is heated by feeding it through a preheater and into a nip formed between the imaging member and a pressure member, either or both of which can also be heated. The nip is maintained at a high pressure by forcing a high durometer synthetic transfix pressure member against the imaging member. As the imaging member rotates, the heated print medium is pulled into and through the nip and is pressed against the deposited ink image by the opposing surfaces of the transfix pressure member and the image member.
The high pressure conditions within the nip compresses the print medium and ink together, spreads the ink droplets, and fuses the ink droplets to the print medium. Heat from the preheated print medium heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print medium. When the print medium leaves the nip, stripper fingers or other like members peel it from the printer member and direct it into a media exit path.
To optimize image resolution, the conditions within the nip must be carefully controlled. The transferred ink drops should spread out to cover a specific area to preserve image resolution. Too little spreading leaves gaps between the ink drops while too much spreading results in intermingling of the ink drops. Additionally, the nip conditions must be controlled to maximize the transfer of ink drops from the image member to the print medium without compromising the spread of the ink drops on the print medium. Moreover, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion thereby optimizing printed image durability. Thus, the temperature and pressure conditions must be carefully controlled and must be consistent over the entire area of the nip.
The necessary pressure and temperature within the nip are a function not only of the particular ink, but also of the rate at which images are transferred from the imaging member to the print medium. In other words, spreading and transfer of ink is a function not only of the pressure and temperature conditions within the nip, but also of the duration that the ink is within the nip. Thus, as the process speed is increased, one or more of the pressure within the nip, the temperature within the nip, and the nip width (the in-process dimension of the nip) must increase to provide desired image quality.
The nip width is a function of the diameters of the image member and the transfix member. Thus, increased process speed is enabled by increased image member and transfix member diameter. Increasing the diameter of the image member and the transfix member, however, requires a larger frame. Nip width can also be increased, without increasing the diameter of the image member and the transfix member, by increasing the pressure within the nip thereby flattening the surfaces of the rolls within the nip. Accordingly, the applied load on the transfix pressure member in certain printer systems is increased from 1,100 pounds up to about 4,000 pounds to provide consistent image quality at increased speeds.
Accordingly, in order to achieve the uniform high pressures needed for high speed imaging, particular attention must be given to the manner in which the transfix pressure roller is manufactured. By way of example, force is applied to the imaging member and the transfix pressure roller at the outer edges of the rollers. Consequently, application of the high pressures needed for high speed imaging results in deformation of the transfix roll with the end portions of the transfix roll positioned closer to the axis of rotation of the image drum than the center portion of the transfix roll. The deformation of the transfix roll caused by application of force only at the outer ends of the transfix roll results in an undesired pressure profile for a transfix roll with a flat profile in the cross-process direction wherein the pressures at the outer edges of the process path are higher than the pressure in the middle portion of the process path. One approach to correcting this issue is to form a transfix roll with a crowned profile.
A “crowned profile” is a profile wherein the diameter of the transfix roll at the middle of the process path is larger than the diameter of the transfix roll at the outer portions of the process path. Transfix rolls with crowned profiles provide a desired image quality, roll life, and acceptable cost. Optimal performance of the crowned transfix pressure component, however, is achieved by adhering to carefully controlled manufacturing tolerances of small magnitude.